Here's how the Blades could line up for the big game...
1. CB: Phil Jagielka
The Blades veteran could make his first start for the club in 12 years, after joining on a free transfer earlier in the summer. Photo: Christopher Lee
2. CB: Joel Lynch
The left-footed defender is currently on trial with the Blades, and Wilder will be eager to give him a chance to shine against Los Verdiblancos. Photo: Jack Thomas
3. RWB: George Baldock
The player has been linked with a move to Celtic this week, but he's set to start for United on Friday night. Photo: Nigel Roddis
4. LWB: Enda Stevens
The Republic of Ireland international is tipped to make a big impact in the Premier League next season, and it'll be interesting to see how he fairs against La Liga opposition. Photo: Ross Kinnaird